Enlargement of thyroid gland (goitre) may be associated with

Goitre refers to enlargement of thyroid gland. It may be associated with hyperthyroidism or hypothyroidism.
Graves’ disease is an autoimmune disorder in which the person produces antibodies that mimic the action of TSH. The antibodies continually stimulate the thyroid gland to grow and produce excess of thyroid hormones.
In some places in the world, dietary iodine intake is inadequate and the resultant low level of thyroid hormone in the blood stimulates secretion of TSH, which causes thyroid gland enlargement.